Summary. Five pregnant beagle bitches were treated with 2·5 mg mifepristone/kg body weight, twice a day, for 4·5 days starting at Day 32 of gestation. Results of fetal ultrasonography and assay of serum progesterone concentrations every 2–4 days were compared to those in 5 control bitches. Mifepristone resulted in a premature (P < 0·01) termination of pregnancy (36 ± 1 vs 65 ± 1 days), without side effects. The antiprogestagen also caused progesterone to decline to < 1 ng/ml by Day 40–45 after the preovulatory LH peak (vs 64–67 days in controls) and reduced (P < 0·05) mean concentrations on Days 34–50 (2·2 ± 0·5 vs 6·3 ± 0·3 ng/ml). The results suggest that antiprogestagen therapy is a safe means to terminate unwanted pregnancy in dogs, and that luteal function in pregnant bitches is dependent on luteotrophic support that is blocked by antiprogestagen treatment, directly or indirectly, due to termination of pregnancy.
Keywords: progesterone; pregnancy; abortion; antiprogesterone; corpus luteum; luteolysis; dog